The present invention relates to a transmission system using optical fibers and, more particularly, to a long-distance, high-capacity optical communication system using return-to-zero (RZ) lightwave pulses and optical amplifying repeaters.
Buttressed by developments of optical amplifying techniques, fiber optic communication technology has made rapid-paced progress toward ultra-long-distance communication, now allowing implementation of a transpacific communication system without the need of using regenerative repeaters. A transoceanic transmission system at a bit rate or transmission rate of about 5 Gbit/s is now feasible through dispersion management of optical fibers. At increased transmission rate above 10 Gbit/s, however, a transmission system using the non-return-to-zero (NRZ) pulse suffers serious degradation of transmission characteristics by the wavelength dispersion characteristic and nonlinear optical effect of optical fibers, imposing severe limitations on the realization of a high-speed, high-capacity transmission system. In recent years, an optical soliton communication system has been in the limelight as a system which will break the bottleneck in the speedup of transmission by the wavelength dispersion characteristic and the nonlinear optical effect.
The optical soliton communication system is a system that positively utilizes the wavelength dispersion characteristic and nonlinear effect of optical fibers which are major factors to the degradation of characteristics of the conventional transmission systems and that transmits optical short pulses intact by balancing optical pulse width expansion owing to the wavelength dispersion by the optical fibers and pulse width compression based on the nonlinear optical effect. The soliton lightwave pulse, which stably propagates without changing its shape and size, is based on the assumption that it is free from losses by the transmission medium; since ordinary optical fibers produce losses, however, the light intensity becomes attenuated with distance, resulting in the nonlinear optical effect being lessened and becoming unbalanced with the dispersion effect. To realize fiber-optic transmission with apparently no loss while keeping the light intensity at a certain value, it is necessary, therefore, that losses by optical fibers be compensated for by optical amplifiers. In the case of using the optical amplifier as a repeater, it is possible to accomplish soliton communication with practically no waveform variations of lightwave pulses like ideal soliton pulses, by setting the average power between repeaters and the average dispersion of optical fibers to soliton conditions.
In the optical soliton communication, however, optical amplifier noise affects the timing jitter of lightwave pulses at the receiving end and eventually deteriorates the transmission characteristic. That is to say, soliton lightwave pulses with noise superimposed thereon undergo random fluctuations of their light intensity and slightly shift in shape from an ideal soliton lightwave pulse, causing fluctuations in the shift amount of the carrier frequency by the nonlinear optical effect. Since this is repeated for each repeater, the time of arrival of lightwave pulses randomly fluctuates during their propagation in optical fibers each having a limited dispersion value, incurring the timing jitter at the receiving end. This phenomenon is called the Gordon-Haus effect, which is a major limiting factor to the transmission characteristic of the optical soliton communication.
FIG. 18 shows the results of computer simulations carried out on 6000 km transmission of 20 Gbit/s soliton lightwave signal over a conventional optical soliton transmission system with optical amplifying repeaters placed at 30 km intervals. FIG. 18(a) is an eye diagram of the waveform of the lightwave signal obtained after the 6000 km transmission and FIG. 18(b) an eye diagram of the lightwave signal after converted by an optical receiver into an electrical signal and then passed through a low-pass filter. It is seen from FIG. 18(a) that the time of arrival of the lightwave signal largely varies owing to the optical amplifier noise, causing a great timing jitter. The bit error rate can be obtained by determining, through stochastic techniques, if the signal level is "1" or "0," depending on whether it is larger or smaller than a certain threshold value at a proper time in the eye diagram of FIG. 18(b). In this instance, since the timing jitter is large, the bit error rate is about 10-5 even if the most appropriate time of determination and the most appropriate threshold value are set. Thus, the prior art cannot ever accomplish a bit error rate which is required in practice, for example, 10.sup.-12. As described above, even if the optical soliton transmission system is employed, the distance of transmission is limited by the Gordon-Haus effect in an ultrafast optical transmission system, for example, in a 20 Gbit/s class.